Damage to the weight-bearing surfaces of the knee as a result of arthritis and/or trauma can be isolated to one compartment, or can be global (involving two or more of the three compartments of the knee). The three compartments of the knee are the patellofemoral compartment, the medial compartment and the lateral compartment. When damage to the knee results in a change in the shape of the underlying bone of the knee, the damage is permanent. Surface replacement procedures such as a total knee arthroplasty (TKA) and a unicompartmental knee arthroplasty (UKA) are typical corrective measures for this damage.
The natural biomechanics of the knee places the weight-bearing axis in a position to allow approximately 60% of the weight bearing by the medial compartment and 40% by the lateral compartment of the knee. This weight-bearing axis is measured in the frontal plane and is a line drawn from the center of the femoral head to the center of the ankle. Therefore, this line must fall slightly to the medial side of the center of the knee to accomplish this weight-bearing distribution. Load distribution across the knee corresponds to the relative surface area of each compartment. That is to say, the medial compartment is generally 60% of the total bearing surface area of the knee. The body distributes this weight optimally to maintain a certain amount of load per square inch of surface area. When this load exceeds a certain level in one or the other compartment, overload in that compartment occurs and damage ensues. The damage begins with articular cartilage wear and can progress to flattening of the condyle with its resultant shape change in the condyle.
When only one compartment in the knee has been damaged permanently, a unicompartmental knee arthroplasty or UKA is used to replace the damaged bone surface. This type of prosthesis has several common failure modes: 1) the side with the UKA fails due to poly wear or interface failure; 2) the opposite compartment fails due to a substantial increase in arthritis; and 3) the femoral component impinges on the patella creating an impingement syndrome. Each of these failures can be attributed to misalignment of the implant. Therefore, there is great need when performing a UKA to implant the device such that the proper mechanical axis within the knee is recreated. Since every lower extremity is unique, the mechanical axis must be uniquely reproduced for the best chance at long-term success. When installed in alignment with the proper axis, the UKA survival rate is greatly improved.
Total Knee Arthoplasty or TKA is a surgical procedure wherein both the lateral and medial compartments are replaced. The proper mechanical axis for these procedures is also important to the long-term success of the implant.
Several prior art devices have been used for the purpose of establishing an axis for partial and total knee replacement surgery. One procedure involves forcing a metal rod into the end of the thighbone. The surgeon then uses this rod to estimate the proper angle for cutting the bone and installing the prosthesis. This procedure has numerous disadvantages including inaccurate cutting of the bone resulting in incorrect placement of the prosthesis and blood clots that may occur from forcing the rod into the marrow of the thighbone to estimate the mechanical axis.
An additional method for aligning the bones for a knee replacement surgery is a device called an intramedullary guidance system. This device takes measurements of the leg and calculates the proper alignment. However, the measurements tend to be inaccurate because they are taken from the somewhat deformable tissue covering the bone rather than the more rigid bone itself. As a result, the alignment measurements vary based on the amount of tissue covering the bone.
Once the axis is estimated using one of the techniques described above, a cutting guide is secured to the bone, which requires multiple incisions. The securing process may weaken the bone and increase the recovery time.
An additional factor in the placement of a knee prosthesis is the natural amount of ‘play’ in the joint. This ‘play’ is defined as the natural amount of motion between the two bones of the joint allowed by the ligaments. In a knee joint with a lot of “play,” incongruous joint surfaces will still allow full flexion and extension. On the other hand, a knee joint with very little ‘play’ must have perfectly shaped joint surfaces to allow full flexion and extension. One job of the surgeon during implantation of either a TKA or a UKA is the need to recreate the patient's normal amount of joint ‘play’. This means the appropriate amount of ligament release and implant size must be used to allow for both the appropriate amount of ‘play’ and correction of the mechanical axis. The current known methods do not provide a method of determining the natural play of a joint.
Therefore there is a need for methods and apparatuses for the alignment of a leg for surgery or other medical attention that address deficiencies in the art, some of which are discussed above.